gccrs: lower: Correctly lower parenthesized types

gccrs: lower: Correctly lower parenthesized types

This is useful for handling multiple trait bounds, and required for better handling of auto traits.

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-type.cc (ASTLoweringType::visit): Add implementation for
	ParenthesizedType.
	* hir/rust-ast-lower-type.h: Declare that new visitor.

gcc/testsuite/ChangeLog:

	* rust/compile/auto_traits1.rs: New test.